Wind turbine with counter rotating rotors

ABSTRACT

A first capture unit includes a first turbine rotor having a first hub. A second capture unit includes a second turbine rotor having a second hub. The rotors counter-rotate independently. A first electric generator includes a first rotor fixed to the first turbine rotor and a first stator fixed so as to face the first rotor. A second electric generator includes a second rotor having a rotor fixed to the second turbine rotor and a second stator fixed so as to face the second rotor. Power electronic means control the electric currents produced by the first and the second stators of the first and the second electric generators independently of each other thus regulating the rotational speed of the first and second turbine rotors.

FIELD OF THE INVENTION

The invention relates to a device for capturing wind energy in order toproduce electrical energy.

BACKGROUND OF THE INVENTION

Devices for capturing the energy supplied by the wind, or wind turbines,comprising a vertical mast, a nacelle mounted so that it can rotateabout a vertical axis on the upper part of the mast, and at least onecapture unit carried by the nacelle, are known. The unit for capturingthe wind energy comprises at least one turbine rotor consisting of a hubmounted so that it can rotate on the nacelle about an approximatelyhorizontal axis and at least two blades (generally two or three blades)fixed to the hub in approximately radial directions.

The nacelle, which is generally streamlined, is oriented, automaticallyor by command, in such a way that the horizontal axis of rotation of thehub is directed into the wind and so that the turbine rotor is rotatedat a certain speed which depends on the wind speed.

The energy recovered by the turbine rotor of the wind turbine can beused in different ways and, in particular, this energy can be convertedinto electrical energy which can be used locally at the site of the windturbine or sent to a distribution grid.

In this case, the capture unit of the wind turbine also constitutes aunit for producing electrical energy and comprises an electric generatorhaving at least one rotor secured to the turbine rotor and at least onestator fixed to the nacelle.

A first problem encountered in the case of units for capturing windenergy and producing electrical energy relates to the need to make therotor the electric generator rotate at a sufficiently high speed, byrotation of the turbine rotor. To achieve that, it is generallynecessary to use mechanical step-up gearing between the turbine rotorand the generator rotor. Such a device makes the wind turbine morecomplicated to construct and to maintain.

It has also been proposed for at least two turbine rotors mounted on oneand the same axis and rotating in opposite directions to one another tobe combined for driving the electric generator. In this case, it isnecessary to provide means of mechanical connection between the twoturbine rotors so as to drive and to regulate the rotational speed ofthe rotor of the electric generator.

Such mechanical connection devices are complex and considerably increasethe size of the functional part of the wind turbine.

When two contra-rotating turbine rotors are used, a first turbine rotoris directed into the wind and the second turbine rotor, which follows onfrom the first turbine rotor in the direction of the wind, uses at leastsome of the recoverable energy of the wind which was not captured by thefirst turbine rotor.

The devices for mechanical connection between the turbine rotors do notgenerally allow the two turbine rotors to operate ideally, whatever thewind speed, that is to say do not allow operation such that the combinedenergy produced over a given period, for example over a year, is asclose as possible to the maximum combined recoverable amount of energy.

In other words, there is not yet known any means that allows theoperation of the first and of the second contra-rotating turbine rotorsto be optimized according to the wind speed.

When the wind speed increases, above and beyond the wind turbinestart-up speed, at a certain wind speed aerodynamic stall occurs or isbrought about, providing some regulation of the operation of the windturbine. It is necessary to provide mechanical means, for exampledevices for adjusting the pitch of the blades of the turbine rotors ofthe wind turbines, in order to stall or feather the turbine rotors, thatis to say bring the blades into a position not subjected to the wind,under desired conditions. These mechanical devices are complex and mayincrease the risks of breakage and wear of the wind turbine in service.

Furthermore, the driving of the rotor or of the electric generators, offone or more turbine rotors, entails the use of mechanical means whichincrease the size of the capture units, particularly in the direction ofthe axis of rotation of the turbine rotors and of the generator rotors.

It is therefore preferable to provide a direct connection between theturbine rotor and the generator rotor. This type of drive cannot be usedin the case of electric generators of current type. The use of make itpossible to make an easier connection with the turbine rotor of the windturbine and to increase the compactness of the capture unit of the windturbine in the axial direction. However, such generators of the discoidtype have never been used in wind turbines with contra-rotating turbinerotors.

BRIEF DESCRIPTION OF THE INVENTION

The object of the invention is therefore to provide a device forcapturing wind energy to produce electrical energy, comprising avertical mast, a nacelle mounted so that it can rotate about a verticalaxis on the upper part of the mast and at least one capture unitcomprising at least one turbine rotor consisting of a hub mounted sothat it can rotate on the nacelle about an approximately horizontal axisand at least two blades fixed to the hub in approximately radialdirections and an electric generator having at least one rotor connectedto the turbine rotor such that it is driven in rotation by the turbinerotor and at least one stator fixed to the nacelle, it being possiblefor this device to be obtained compactly while at the same timeexhibiting high installed power and making it possible to increase theamount of energy produced during a reference period, for example overthe course of one year.

To this end, the capture device according to the invention comprises afirst capture unit and a second capture unit, these respectivelycomprising a first and a second turbine rotor which contra-rotate andare arranged one on each side of the vertical axis of the mast, the hubsof which are mounted to rotate independently of one another aboutaligned axes and a first and a second electric generator produced indiscoid shape and each comprising:

at least one rotor having at least one disc-shaped part secured to thecorresponding turbine rotor,

at least one stator having at least one disc-shaped part facing therotor, and

power electronics means associated with the generator to allow the speedof the rotor to be regulated independently on each of the capture units.

The invention also relates to a method for regulating the capture devicein order to optimize its operation so as to produce the maximum combinedenergy over a reference period.

In order to make the invention easy to understand, a device forcapturing wind energy and for producing electrical energy according tothe invention and its optimized use for producing a maximum combinedamount of energy over a reference period will now be described by way ofexample with reference to the appended figures.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 is a view in axial section of the capture device according to theinvention.

FIG. 2 is an enlargement of part of FIG. 1, showing the electricgenerator of one of the capture units.

FIG. 3 is a diagram showing, as a function of wind speed, the powersupplied by each of the capture units and the total power supplied bythe device, and the distribution over time of the wind speeds over areference period of one year.

FIG. 4 is a diagram showing, as a function of wind speed, the annualenergy production of the capture device according to the invention.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 shows a capture device according to the invention denoted overallby the reference 1.

FIG. 1 depicts only the upper part of the mast 2 of the capture devicecarrying the nacelle 3 at its upper end, via a bearing 4 which allowsthe nacelle 3 to be mounted such that it can rotate on the upper part ofthe mast 2 about the vertical axis 5.

The mast 2, only the upper part of which has been depicted in FIG. 1,may be very tall, for example of the order of 40 m high, the lower partof the mast being fixed into a solid footing in the ground at the windenergy production site.

A streamlined casing 6, of profiled shape, is fitted around thestructure of the nacelle 3. The shape of the streamlining 6 is chosen,in particular, to meet requirements pertaining to the visual impact ofthe wind turbine.

On each side of the vertical axis 5 of the mast 2, about which it ismounted to rotate, the nacelle 3 has two extensions 3 a and 3 b, on eachof which is mounted, respectively, via bearings 7 a and 7 b, a first hub8 a of a first turbine rotor 10 a of the capture device and a second hub8 b of a second turbine rotor 10 b.

The first turbine rotor l0 a is for driving the rotor of a firstelectric generator 9 a, and the second turbine rotor 10 b is for drivingthe rotor of a second electric generator 9 b of the capture device. Thefirst turbine rotor 10 a and the first generator 9 a constitute a firstcapture and electrical-energy-production unit and the second turbinerotor 10 b and the second generator 9 b constitute a second capture andelectrical-energy-production unit of the capture device 1.

The bearings 7 a and 7 b for the rotary mounting of the hubs 8 a and 8 bhave an approximately horizontal common axis 11 which is pointed in theoverall direction in which the wind blows while the wind turbine is inoperation. The wind has been depicted in the conventional way by arrows13 upstream of the first turbine rotor 10 a and 13′ upstream of thesecond turbine rotor 10 b.

In general, the turbine rotors 10 a and 10 b and the generators 9 a and9 b constituting the capture units are mounted on the nacelle 3 insymmetric arrangements with respect to the vertical axis 5 of the mast2.

The first turbine rotor 10 a is placed facing into the wind depicted bythe arrows 13 and the second turbine rotor 10 a is placed on the leeside of the residual wind which has passed through the first turbinerotor, this residual wind being depicted by the arrows 13′, the twoturbine rotors 10 a and 10 a being aligned in the direction of the wind.The turbine rotors 10 a and 10 b are mounted so that they can rotate onthe nacelle 3, entirely independently of one another, there being nomeans of mechanical connection between the two turbine rotors.

Each of the turbine rotors 10 a and 10 b consists of the correspondinghub 8 a or 8 b and of a set of respective blades 12 a or 12 b fixedrigidly in approximately radial directions to their hub 8 a or 8 b.

Each of the rotors may, for example, have two or three radial blades set180° or 120° apart about the axis 11 of rotation of the hubs.

As visible in the central part of FIG. 1, the blades 12 a and 12 b havea profiled shape in cross section on a plane parallel to the axis ofrotation of the turbine rotor.

The profiles of the blades 12 a and 12 b are the reverse of each other,which means that the wind causes the two turbine rotors to rotate inopposite directions. The two turbine rotors are therefore said to becontra-rotatory. The circular arrows 14 and 14′ have been used to,depict the direction of rotation of the first turbine rotor 10 a and ofthe second turbine rotor 10 b, respectively.

Each of the electric generators 9 a and 9 b is produced in discoid formand comprises two rotors each of which rotates as one with thecorresponding turbine rotor, and a double stator fixed to part of thenacelle 3.

The two electric generators 9 a and 9 b, which are arrangedsymmetrically with respect to the axis 5, are produced in the same waywhich means that only the generator 9 a of the first capture unit willbe described in detail, with reference to FIG. 2.

The generator 9 a, produced in discoid form, comprise two stators 15 and15′ of annular overall shape and comprising a part 16 or 16′ in the formof a flat disc carrying, on its external face facing toward the stator,permanent magnets 17 or 17′. The discs 16 and 16′ of the rotors 15 and15′ are secured to an annular hollow rotor body containing a laminatedcore consisting of a stack of laminations. The rotor 15 is fixeddirectly on the hub 8 a by screws 19 which also fix the rotatinginternal part of the bearing 7 a of the rotary mounting of the hub 8 aand of the turbine rotor 10 a.

The fixed outer part of the bearing 7 a is secured to a part of thenacelle 3 to which is also fixed the stator 18 of annular overall shapewith two flat discoid faces placed facing flat discoid faces of thestators 16 and 16′ carrying the permanent magnets 17 and 17′.

The two rotors 15 and 15′ are secured together by screws which clamp thediscs 16 and 16′ of the stators against a multi-part peripheral ring 20.The rotors are held in the axial direction by thrust bearings associatedwith the rotary bearing 7 a and by a double thrust bearing 21 whichholds the rotor in the axial direction and in two opposite directions.

The stator 18 has two parts facing, respectively, the rotor 16 and therotor 16′ and each of which consists of a laminated core in which aremounted coils facing the permanent magnets 17 and 17′ of the rotors 16and 16′.

The coils of the stator 20 are connected by electrical conductors tomeans of connecting the generator to a user line for the currentproduced. The stator coils are also connected to a box 22 secured to thenacelle 3 and containing power electronics for controlling the electricgenerator and regulating the rotational speed of the rotors 15 and 15′.

Of course, the second electric generator 9 b is connected in the sameway as the first electric generator to power electronics which may belocated in the box 22, so that the first and second electric generatorscan be controlled entirely independently and so that the speed of therotor of the first generator and of the first turbine rotor and of therotor of the second generator and of the second turbine rotor can beregulated entirely independently.

The method for regulating the capture device according to the inventionby regulating the speed of the rotors of the generators and of theturbine rotors of the two capture units of the device according to theinvention will now be described with reference to FIG. 3 and FIG. 4.

FIG. 3 represents, in the form of a curve 23, the number of hours (alongthe ordinates axis) in a reference period of one year, for which thewind to which the capture device according to the invention is subjectedhas a speed which is indicated along the abscissas axis. In reality,each of the points giving a number of hours with a certain wind speedcorresponds to a range of speeds of an amplitude of one meter/second.

The curve 23 represents the distribution of wind speeds over the courseof the year at the site at which the wind turbine is erected. The curve23 has an initial point to the left in FIG. 3, corresponding to a speedof the order of 3 m/second which is the wind speed needed to start thewind turbine. The power supplied respectively by the first capture unitand by the second capture unit of the device according to the invention,as a function of wind speed, is also given in FIG. 3, in the form of thecurves 24 and 25.

Finally, the curve 26 represents the total power supplied by the device,that is to say the sum of the powers supplied by the first and by thesecond capture units of the device.

As visible in FIG. 3, the two capture units start up at a wind speed ofthe order of 3 m/s and the turbine rotors of the capture units rotate ata speed which increases as the wind speed increases. Correspondingly,increasing amounts of power are produced by each of the capture units.

In a first zone A, the speed of the rotary part of the first captureunit facing into the wind increases until it reaches a speed thatcorresponds to the onset of regulation by aerodynamically stalling therotary part of the first capture unit. The speed at which aerodynamicstall begins is around 9 m/second. This stall may be commanded orobtained automatically when the wind speed reaches the limit determinedby the characteristics of the first capture unit.

Under the operating conditions in zone A, the first capture unit worksat its maximum efficiency, the coefficient of power CP or Betzcoefficient being at a maximum. The coefficient of power or Betzcoefficient is defined as the ratio of energy recovered to maximumrecoverable energy which represents about 60% of the kinetic energy ofthe wind.

Because the first capture unit is operating at maximum efficiency, thesecond capture unit has only a fraction of the recoverable kineticenergy available to drive it, this fraction representing for examplefrom 50 to 80% of the energy absorbed by the first capture unit.

FIG. 3 represents the scenario in which the second capture unit has only50% of the energy captured by the first capture unit available to it.The rotational speed of the rotary part of the second capture unit isadapted to suit the wind speed so that this second capture unit works atmaximum CP coefficient.

In zone A, the total recovered power is, in this instance, equal to oneand a half times the power captured by the first capture unit. The gainin power due to the second capture unit is therefore, in this instance,about 50% in zone A.

After zone A, the graph of FIG. 3 represents a zone B ranging from thepoint of aerodynamic stall of the first capture unit (for a wind speedof the order of 9 m/second) up to the point of aerodynamic stalling ofthe second capture unit (for a wind speed of the order of 11 m/second).

In this zone B, the rotary part of the first capture unit begins toexperience aerodynamic stall and the stronger the wind becomes, the morethe rotary part of the first capture unit experiences stall; what thismeans is that the residual kinetic energy that can be recovered by therotary part of the second capture unit increases up to the point atwhich the rotary part of the second capture unit stalls at a speed (forexample 11 m/second) which is dependent upon the characteristics of thesecond capture unit. In zone B, the second capture unit is operating atmaximum CP.

In the next zone, C, the two capture units are operating in conditionsof increasing stall, the first capture unit reaching maximum stallingbefore the second capture unit which means that the gain in powercontributed by the rotary part of the second capture unit progresses inzone C from 50%, for the speed at the onset of stall, to 100% for thewind speed (around about 14 m/second) at which the rotary parts of bothcapture units are experiencing aerodynamic stall.

Thereafter, up to the highest wind speeds (zone D), both capture unitsare making the same contribution to the total power supplied by thecapture device.

It is therefore obvious that the speed regulation achieved independentlyon the rotary parts of each of the capture units makes it possible, forany wind speed, to optimize the capture device so that the powersupplied by this device will be as high as possible, given the amount ofwind energy that can be recovered.

The speeds of the rotary parts of the capture units are regulated by thepower electronics associated with the electric generator of thesecapture units.

FIG. 4 represents, in the form of a curve 27, the energy produced in thereference period of one year by the first capture unit as a function ofthe wind speed and, in the form of a curve 28, the energy producedannually by the entire device consisting of the first and second captureunits, as a function of wind speed.

The curves 27 and 28 are obtained from the curve 23 and the curves 24and 26 respectively, by multiplying the powers supplied by the number ofhours corresponding to that wind speed.

The use of a second capture unit mounted on the nacelle of the windturbine after the first capture unit makes it possible to increase theenergy recovery by about 60% to 70% compared with the use of a singlecapture unit having a rotary part of the same diameter and with the sameaerodynamic characteristics as the rotary part of the capture unit usedin addition.

Regulating the speed of the rotary parts of the capture units makes itpossible to optimize the recovery of energy on each of the capture unitsand, in particular, to make the second capture unit operate in a waywhich is optimum for the recovery of the recoverable energy which is notcaptured by the first capture unit.

The device and the method according to the present invention thereforemake it possible to increase the installed power of a device forcapturing wind energy by using a first and a second capture unit oneafter the other and to increase the energy produced over a referenceperiod by regulating the first and second capture units.

The device according to the invention also is very compact in spite ofthe use of two capture units arranged one after the other. Thiscompactness is obtained by virtue of the use of discoid electricgenerators.

Furthermore, the capture device according to the invention has nocomplex and fragile mechanical parts and the speed of the rotary partsof the capture units is regulated entirely by electronic means.

The invention is not strictly limited to the embodiment which has beendescribed.

Thus, the turbine rotors and the electric generators of the captureunits may be produced in a different form.

The turbine rotors of the capture units may have any number of blades,the length of which may be chosen from a vast range.

The electric generator may have one single rotor and one single statoror, on the other hand, one or more units themselves comprising one ormore rotors and one or more stators.

The power electronics for controlling the electric generators andregulating the speed of rotors associated with the turbine rotors may beproduced by any means known to those skilled in the art.

The invention applies to the manufacture and operation of any windturbine used for producing electrical current.

What is claimed is:
 1. A device for capturing wind energy to produceelectrical energy, comprising: a vertical mast; a nacelle mountedrotatably about a vertical axis on the upper part of the mast; a firstcapture unit including a first turbine rotor having a first hub and atleast two first blades fixed to the first hub in generally radialdirections, said first hub being mounted rotatably about a generallyhorizontal axis on the nacelle; a second capture unit including a secondturbine rotor having a second hub and at least two second blades fixedto the second hub in generally radial directions, said second hub beingmounted rotatably about the generally horizontal axis about which saidfirst hub is rotatably mounted so that said first turbine rotor and saidsecond turbine rotor rotate independently of each other and said bladesof said second rotor are oriented such that said first rotor and saidsecond rotor counter rotate; a first electric generator of discoid shapeincluding a first rotor having one disc-shaped rotor part fixed to thefirst turbine rotor and a first stator having one disc-shaped statorpart fixed on the nacelle so as to face the rotor part of the firstrotor; a second electric generator of discoid shape including a secondrotor having one disc-shaped rotor part fixed to the second turbinerotor and a second stator having one disc-shaped stator part fixed onthe nacelle so as to face the rotor part of the second rotor; and powerelectronic means for controlling the electric currents produced by saidfirst and said second stators of said first and said second electricgenerators independently of each other thus regulating the rotationalspeed of said first turbine rotor and said second turbine rotor.
 2. Thewind energy capturing device of claim 1 wherein the power electronicmeans regulates the rotational speed of the first and second rotorsindependently of each other at an optimum value depending on the windspeed for optimizing the energy and power produced by the device.
 3. Thewind energy capturing device of claim 2 wherein the power electronicmeans operates so that, in a first zone of operation A of a plot ofpower supplied by said first and said second capture units as a functionof the wind speed, for wind speeds ranging from the capture unitstart-up speed to a speed at which the rotating part of the firstcapture unit is aerodynamically stalled, the first capture unit and thesecond capture unit are operated under conditions of maximum efficiency,and the speed of the rotating parts of the capture units is thenregulated in order to regulate the aerodynamic stall of the rotatingpart of the first capture unit and cause the residual energy to berecovered by the rotating part of the second capture unit, so that theincrease in power and energy produced by the second capture unitgradually reaches the value of the power and of the energy recovered bythe first capture unit, for increasing wind speed.
 4. The wind energycapturing device of claim 3 wherein the power electronic means operatesso that the increase in power and energy produced by the second captureunit changes progressively, after the onset of stall in the rotatingpart of the first capture unit, from 50% to 100% of the power and energyproduced by the first capture unit.